1. Field of the Invention
This invention relates to a deceleration control apparatus which can control a two-cycle engine of the spark ignition type for a vehicle, in which a fresh air/fuel mixture in a combustion chamber can be self-ignited at an ignition timing preferable for operation of the engine at least upon low load operation, so that, when engine sudden deceleration requirements are satisfied, the combustion condition is automatically changed over from an activated heat atmosphere combustion condition to an ordinary combustion condition to allow sudden deceleration of the engine.
2. Description of Background Art
In a conventional two-cycle engine of the spark ignition type for a motorcycle or a like vehicle, an exhaust port and a scavenging port which are each opened or closed by a piston are formed on an inner periphery of a cylinder bore, and a fresh air/fuel mixture pre-pressurized in a crank chamber is supplied into the cylinder chamber through the scavenging port while combustion gas in the cylinder chamber is exhausted through the exhaust port. The fresh air/fuel mixture supplied into the cylinder chamber is ignited by means of an ignition plug. Since it is necessary during idling to supply a fresh air/fuel mixture by more than a certain amount into the cylinder, a throttle valve in an intake air passage must be opened to a certain valve opening, particularly by more than about 10% of the full throttle amount.
Further, in such a conventional two-cycle engine of the spark ignition type as described above, if the exhaust port is made large in order to set the output power and the efficiency in a high speed, high load operation region, then, in a low load operation region, the amount of unburnt hydrocarbons in the exhaust gas is increased by blow-by of fresh air or unstable combustion, resulting in deterioration of the fuel efficiency.
In order to eliminate this problem, the present inventors developed a two-cycle engine of the spark ignition type wherein an exhaust control valve is actuated to an exhaust opening ratio in response to the engine speed and the throttle valve opening. The invention controls the in-cylinder pressure when the exhaust opening is closed by the piston at least in a low load operation region so that fresh air in a combustion chamber can be self-ignited at an ignition timing preferable for operation of the engine as disclosed in Japanese Patent Application No. Heisei 5-187488.
The combustion wherein the ignition timing preferable for operation of an engine is controlled positively to cause activated heat atmosphere combustion to take place in this manner is hereinafter referred to as AR combustion.
In a two-cycle engine of the spark ignition type in which AR combustion can take place, as shown in FIG. 6, the engine can sufficiently make effective use, in a low load operation region in which the throttle valve opening .theta..sub.th is restricted, of heat energy included in burnt gas in the last cycle to activate a fresh air/fuel mixture in a combustion chamber to cause the engine to operate in a condition approximate to complete combustion. The two-cycle engine, therefore, has a higher output power characteristic than that in an ordinary combustion condition in which irregular combustion is involved. As a result, where the engine is utilized for driving a vehicle or the like, there is a disadvantage in that, even if the throttle valve opening .theta..sub.th is reduced by a great amount while the engine is operating in a high speed, high load operation condition, the effect of a so-called engine brake by a sudden reduction of the output power of the engine cannot be anticipated sufficiently.